I just upgraded from MW 1.18 to 1.21 (no other changes). My user page was perhaps complex to render, containing large tables and repeated use of an image in the cells. However I didn't have any problems loading it in 1.18. Now, in 1.21 it is timing out in PHP (30 seconds). Previously the page would load in under 10 seconds. PHP is 5.3.5 and MySQL 5.0.37.
Topic on Project:Support desk/Flow
Hmm, hard to say... I would now do some debugging, see Manual:How to debug. The debug options (Profiling!) allow you to write a log for page requests. I would activate that with an according StartProfiler.php file and would then request the according page. Then post the resulting output from the debug file here...
I've isolated this to the use of the image which was based on having Instant Commons. I turned Instant Commons off, and now the page loads as normal, except of course that the image is not displayed, but I guess I will just upload a copy locally. Perhaps there was an issue with caching the image request from commons? Though I don't actually have a proper cache anyway since I'm just running this on a local machine and not a public server. I don't know the workings of MW enough to judge if this has any effect.
So, I uploaded a local copy of the image and this appeared to fix the problem for a short while. Then I edited the page, and that caused it to become unresponsive - it did not time out, but would just get stuck in an indefinite loading state. The entire site became unresponsive until I restarted the browser (Firefox), whereupon every time I visited the user page it would become unresponsive again. The rest of the wiki otherwise worked fine. Since I can't really work with the wiki in such a flaky state, I opted to try rolling back to older versions and in fact had to go all the way back to 1.18. So whatever the issue was, was probably introduced in 1.19 and has been carried in the code since then.